I was climbing an adjacent route and witnessed the incident. I do not know the climbers involved; I think they were British climbers.The weather was fair, not windy. My instructor assisted in the evacuation by lowering the injured person and called the helicopter, so explained how the incident occurred. The climber, seconding, climbed past a clipped bolt without unclipping, called for slack and continued upwards. The belayer must have kept giving slack and eventually the climber fell a very long way, with two full rotations and hitting the slab several times. Injuries included a broken arm, shoulder injury, puncture wound, not sure what else but was able to stand up.